I. Field of the Invention
The present invention pertains generally to the field of wireless communications, and more specifically to providing an efficient method and apparatus for reducing voice latency associated with a voice-over-data wireless communication system.
II. Background
The field of wireless communications has many applications including cordless telephones, paging, wireless local loops, and satellite communication systems. A particularly important application is cellular telephone systems for mobile subscribers. (As used herein, the term xe2x80x9ccellularxe2x80x9d systems encompasses both cellular and PCS frequencies.) Various over-the-air interfaces have been developed for such cellular telephone systems including frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95). In particular, IS-95 and its derivatives, such as IS-95A, IS-95B (often referred to collectively as IS-95), ANSI J-STD-008, IS-99, IS-657, IS-707, and others, are promulgated by the Telecommunication Industry Association (TIA) and other well known standards bodies.
Cellular telephone systems configured in accordance with the use of the IS-95 standard employ CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. An exemplary cellular telephone system configured substantially in accordance with the use of the IS-95 standard is described in U.S. Pat. No. 5,103,459 entitled xe2x80x9cSystem and Method for Generating Signal Waveforms in a CDMA Cellular Telephone Systemxe2x80x9d, which is assigned to the assignee of the present invention and incorporated herein by reference. The aforesaid patent illustrates transmit, or forward-link, signal processing in a CDMA base station. Exemplary receive, or reverse-link, signal processing in a CDMA base station is described in U.S. application Ser. No. 08/987,172, filed Dec. 9, 1997, entitled MULTICHANNEL DEMODULATOR, which is assigned to the assignee of the present invention and incorporated herein by reference. In CDMA systems, over-the-air power control is a vital issue. An exemplary method of power control in a CDMA system is described in U.S. Pat. No. 5,056,109 entitled xe2x80x9cMethod and Apparatus for Controlling Transmission Power in A CDMA Cellular Mobile Telephone Systemxe2x80x9d which is assigned to the assignee of the present invention and incorporated herein by reference.
A primary benefit of using a CDMA over-the-air interface is that communications are conducted simultaneously over the same RF band. For example, each mobile subscriber unit (typically a cellular telephone) in a given cellular telephone system can communicate with the same base station by transmitting a reverse-link signal over the same 1.25 MHz of RF spectrum. Similarly, each base station in such a system can communicate with mobile units by transmitting a forward-link signal over another 1.25 MHz of RF spectrum.
Transmitting signals over the same RF spectrum provides various benefits including an increase in the frequency reuse of a cellular telephone system and the ability to conduct soft handoff between two or more base stations. Increased frequency reuse allows a greater number of calls to be conducted over a given amount of spectrum. Soft handoff is a robust method of transitioning a mobile unit between the coverage area of two or more base stations that involves simultaneosly interfacing with two or more base stations. (In contrast, hard handoff involves terminating the interface with a first base station before establishing the interface with a second base station.) An exemplary method of performing soft handoff is described in U.S. Pat. No. 5,267,261 entitled xe2x80x9cMobile Station Assisted Soft Handoff in a CDMA Cellular Communications Systemxe2x80x9d which is assigned to the assignee of the present invention and incorporated herein by reference.
Under Interim Standards IS-99 and IS-657 (referred to hereinafter collectively as IS-707), an IS-95-compliant communications system can provide both voice and data communications services. Data communications services allow digital data to be exchanged between a transmitter and one or more receivers over a wireless interface. Examples of the type of digital data typically transmitted using the IS-707 standard include computer files and electronic mail.
In accordance with both the IS-95 and IS-707 standards, the data exchanged between a transmitter and a receiver is processed in discreet packets, otherwise known as data packets or data frames, or simply frames. To increase the likelihood that a frame will be successfully transmitted during a data transmission, IS-707 employs a radio link protocol (RLP) to track the frames transmitted successfully and to perform frame retransmission when a frame is not transmitted successfully. Re-transmission is performed up to three times in IS-707, and it is the responsibility of higher layer protocols to take additional steps to ensure that frames are successfully received.
Recently, a need has arisen for transmitting audio information, such as voice, using the data protocols of IS-707. For example, in a wireless communications system employing cryptographic techniques, audio information may be more easily manipulated and distributed among data networks by encoding information at a transmitter using a data protocol. In such applications, it is desirable to maintain the use of existing data protocols to minimize changes to existing infrastructure. However, problems occur when transmitting voice using a data protocol, due to the nature of voice characteristics.
One of the primary problems of transmitting audio information using a data protocol is the delay in transmitting information between a transmitter and a receiver. Such a delay, known as latency, may be associated with frame re-transmissions inherent in an over-the-air data protocol such as RLP, or it may be due to the insertion of other information, such as signaling information, into a voice transmission. Delays of more than a few hundred milliseconds in speech can result in unacceptable voice quality. When transmitting data, such as computer files, time delays are easily tolerated due to the non real-time nature of data. As a consequence, the protocols of IS-707 can afford to use the frame re-transmission scheme as described above, which may result in transmission delays, or a latency period, of more than a few seconds. Such a latency period is unacceptable for transmitting voice information.
What is needed is a method and apparatus for minimizing the problems of latency. Furthermore, the method and apparatus should be backwards-compatible with existing infrastructure to avoid expensive upgrades to those systems.
The present invention is a method and apparatus for reducing voice latency associated with a voice-over-data wireless communication system. Generally, this is achieved by altering a data encoding rate, or data rate, of a voice encoder operating within a transmitter.
In a first embodiment of the present invention, audio information, such as human speech, is encoded into data packets, or vocoder frames, by a voice encoder located in a transmitter. Under normal conditions, the voice encoder is operated in a first mode of operation. A processor located within the transmitter measures at least one operational parameter of the transmitter, the at least one operational parameter proportional to a latency between the transmitter and a receiver. If one or more of the operational parameters exceed a predetermined threshold, indicating increased latency, the processor instructs the voice encoder to operate in a second mode of operation.
In one embodiment, the first mode of operation comprises operating the voice encoder at a fixed data rate and the second mode of operation comprises operating the voice encoder at a second fixed data rate.
In another embodiment, the first mode of operation comprises operating the voice encoder at a set of data rates and the second mode of operation comprises operating the voice encoder at a fixed data rate.
In another embodiment, the first mode of operation comprises operating the voice encoder at a fixed data rate and the second mode of operation comprises operating the voice encoder at a set of data rates.
In vet another embodiment, the first mode of operation comprises operating the voice encoder at a first set of data rates and the second mode of operation comprises operating the voice encoder at a second set of data rates.
In general, when latency increases to an unacceptable level, as measured by at least one operational parameter in the transmitter, the voice encoder is operated in a second mode so that less information, on average, is encoded and readied for over-the-air transmission. By encoding less information, fewer frames are produced, thereby allowing a reduction in frames awaiting transmission in a queue in the transmitter. When latency returns to an acceptable level, the voice encoder is operated in the first mode.
In another embodiment, the present invention is an apparatus for reducing voice latency in a voice-over-data wireless communication system, comprising a voice encoder for encoding audio information into data frames at a transmitter, and a processor for determining the value of at least one operating parameter within said transmitter. The at lease one operating parameter is proportional to a latency between said transmitter and a receiver. The processor further compares the at least one operating parameter to a predetermined threshold. The voice encoder is operated in a first mode of operation if the at least one operating parameter is less than the predetermined threshold and operated in a second mode of operation if the at least one operating parameter is greater than the predetermined threshold.